Transient Intermodulation Distortion (TIM) and Zero Global Negative Feedback
So far we have discussed harmonic distortion that arises when non-linear
elements of the amplification path are experienced by the signal. The
other form of distortion that needs discussion is called Transient Intermodulation Distortion or TIM.
One way of reducing harmonic distortion, and improving the THD
specification, is through negative feedback. We have all the painful
experience that happens when someone accidentally puts a microphone too
close to a loudspeaker that is reproducing the amplified signal from the
microphone. The resultant squeal is a result of feedback, positive
feedback in this case. What's happened is that some of the output signal
get into the input, the microphone in this case, and the whole system goes
into oscillation. Oscillation is a sign on an "unstable" circuit.
Negative feedback, where a portion of the inverted output signal is
fed back into the input, can be used to avoid oscillation, provide a more
stable circuit, and to reduce harmonic distortion. Most all amplifiers use
“local” negative feedback to stabilize individual gain stages This is
fine, up to a point.
One can also use negative feedback on the entire amplifier to further
reduce harmonic distortion. In a multi-stage amplifier design, this is
called global negative feedback. Unfortunately global negative feedback
can produce other problems, such as TIM, and while audible, TIM does not
show up on the specification sheet, as THD is typically measured with
continuous sine waves, not dynamic signals such as music.
The problem with global negative feedback is that the feedback signal can
be slightly delayed by the various amplification stages. With a rapidly
changing dynamic multi-frequency signal, i.e., music, these delays and the
global negative feedback can lead to the various frequencies and their
distortion products, modulating each other to produce a series of
transient sum and difference signals. These transient intermodulation
products are called TIM and they sounds like “grunge” between the notes in
a musical passage. Sometimes a reviewer will write about hearing the
silence or space between the notes– a very good way to describe lack of
TIM. Music sounds better and less tiring without "grunge".
The Theta Dreadnaught does not use any global negative feedback, and as a
consequence, sounds very clean with music. You can hear the silence between
the notes. (I am listening to solo acoustic guitar as I write this review,
and it sounds great!)
True Balanced Differential
Noise is the other issue with amplifier designs. Amplifiers will amplify
anything on their input, noise and signal, and pass through noise on their
power supplies or rail voltages. The Dreadnaught II uses a balanced
differential design to minimize noise. Just because an amplifier has XLR
“balanced” inputs does not mean that it is a balanced design, in that some
or all of the amplification stages may be single ended with half of the
signal path connected to ground. The Dreadnaught II is a “true” balanced
design with the use of symmetrical mirror-imaged signal paths all the way
from the balanced XLR input to the speaker output connectors. Single ended
signals from the RCA inputs are “balanced” at the first amplification
stage by cloning and then phase inverting the signal.
The consequence of the balanced designed is that any noise or
“discontinuities” that exist on both signal paths (common mode), are
cancelled. The result is an uncommonly quiet, clean sounding amplifier. I
can put my ear right up to my speakers, with the sound muted, and there is
NO hiss or hum or any sound at all coming from the Dreadnaught II. The
cost of a true balanced design is a more complex and expensive design that
requires literally twice as many components in the amplification circuits.
(You also need to be careful not to short either speaker output to the
chassis or other "ground".)
Listening and Comments
I first hooked up the Dreadnaught II without putting it inside my
equipment cabinet, as frankly, I was a bit skeptical as to how much I'd
appreciate this amplifier. The improvement over my reference system
however, was immediately obvious. The sound had more impact, better
dynamics, and was surprisingly warmer – the mid-bass was definitely better
defined and more obvious. The mid-bass seemed more up front, not just better
defined or “controlled”.
My front speakers are Velodyne DF-661s, speakers that are specified to
have exceptionally low distortion levels, but they also can sound a bit
bright. In the years I have had these speakers, I have tried various
equalizations and even modified the crossover, but to me they always had a
bit of a weak sounding mid-bass. That is until I hooked up the Dreadnaught
II. My wife and I noticed the difference immediately using the opening
passages of "Lord of the Rings, Fellowship of the Ring". Later that evening,
my wife asked how much this wonderful amplifier was, and with some
trepidation, I told her, about $6000. She replied, “I guess that is what a
good amplifier costs.” The next weekend, my older amplifiers moved over to
make room for the Dreadnaught in the equipment cabinet.
Clearly when it came to driving my speakers, the Dreadnaught II did a much
better job of keeping the various frequencies in balance while driving the
complex impedance characteristics that the speakers present to the
amplifier. The two amplifier setups in question, Acurus 200x3 and the 225
watts per channel of the Dreadnaught were of a similar power rating, and
the distortion specifications of the Acurus are probably better than that
of the Dreadnaught, but there was no doubt which sounded better with music
and movie sound tracks.
For most of the listening, I used my B&K Reference 30, and, as it does not
have balanced outputs, RCA-based connections to the Dreadnaught. That
worked fine. I also had a chance to use the Dreadnaught II with the Sunfire
Theater Grand Processor III, using XLR balanced connectors. That was a
really magic combination with even better dynamics, a wider sound stage,
and a more open and transparent high-end. Whether it was the nature of the
Sunfire's processing and electronics, or the use of the balanced inputs to
the Dreadnaught II, or both, I can't say, but there is definitely a new
preamp with balanced outputs in my near-term equipment upgrade plans.
About half way through the review period, I upgraded from the 5x225 watt
configuration with the rear surrounds receiving 2x200 from my Acurus amp,
to the 3x225 and 4x100 7-channel configuration for the Dreadnaught II that
I bought. I'll have to say that even though the setup had less power
available to the surround channels, the latter configuration sounded
better, perhaps because all the amplifier channels had a similar “voice”
and distortion levels, etc. Since the drive level from the preamp for
my rear channels is down by 7 dB relative to the fronts, the fact that the
rear channels have 4 dB less power (100 vs 225 watts) is really not an
issue. And, by getting all seven channels into a single chassis, the
Dreadnaught uses less cabinet space than my previous three-amplifier setup.
(Of course, my old amplifiers still find use for multi-room purposes.)
Video material that we have particularly enjoyed recently includes "Lord
of the Rings, The Two Towers". The opening scene where Gandalf and the Balrog are falling together played at anything near reference levels, will
make a strong impression on any listener. The audio in chapter 5 of
"Chicago" is also fantastic. Another AV treat, is “Pirates of the
Caribbean”. This EX encoded soundtrack is awesome with a good sound
system. Your woofers and well as your subwoofers will get a good workout
with both the theme music and many of the special effects. The bass in the
pirates theme music sounds very solid and well blended between my two 15
inch Velodyne subwoofers and the DF 661s woofers when they were driven by
the Dreadnaught II.
Music is very satisfying with stereo reproduction using the Dreadnaught II
in the Stereo mode, and that is how I currently listen to two-channel
sources such as CDs and the music channels from DISH. The best music
reproduction I have heard however, is with DVD Audio using all 7 amplifier
channels and speakers. Emmylou Harris' "Directors Cut" sounds particularly
good with my Dreadnaught II-based setup.
So what's not to like about the Dreadnaught II? No complaints about the
sound, that's for sure. It will chew up the power if you leave it on, and
for someone who car pools daily in a gas/electric hybrid (Honda Insight),
and is obsessive about recycling, this is an issue that gave me some
pause. Turning it off when not in use seems a good compromise between
great sound and saving the earth.
My only real complaint about the Dreadnaught is its control capability in
the standard configuration – the use of a pulse instead of a level to
select the Operate/Standby and Surround modes. The trouble is that a
pulse is a toggle, and does not allow for a discrete selection of modes.
Toggles are trouble for anyone wishing to write reliable macros.
I got the Operate/Standby selection “working” by assigning a 12V control
output from my preamp to an unused input, and then selecting and
deselecting that input in my “system on” macro. This was neither
satisfying nor reliable. The issue became worse once I started turning
the amplifier off when not in use. Now I needed an Operate/Standby pulse
delayed by 6 seconds to assure that the amplifier was fully on and
listening when the pulse arrived. Furthermore, macros with long delays are
hard to do reliably using an IR-based remote control.
As a consequence, most of my time with the Dreadnaught II I used a
“digital” device to poke the button on the front of the amplifier, but
that was getting old quickly. After experimenting with an electronic
circuit to produce a delayed pulse triggered by a level from my
preamp, I decided that enough was enough, and sent the amp back to Theta
for an upgrade to the RS-232 control module. I also switched to an RF-based
remote for reliable long macros; more about that in another review on
remotes.
Now the problem was that I needed some way of generating the appropriate
RS-232 codes based on the IR (or RF) commands from my remote. There are
several systems available that will do the job, but most are really
designed for use by a professional installer and require the purchase of
not only the device, but their (expensive) programming software as well.
My salvation was a device from Celadon (www.celadon.com). They offer an IR to
RS-232 device, the IRC-38, that comes pre-programmed for specific devices.
If they have the device of interest in their database, then there is no
charge for programming. (Let's just say the Dreadnaught commands are in
their database now.)
Using
the Celadon IRC-38 I have now (photo on the right), not only the Standby/Operate modes
work reliably after power on, but I also have all the surround modes
selection buttons on my remote programmed to select the Surround mode
appropriately. I never used to bother deselecting the surround buss for
stereo listening when it required a manual operation. Now that the
operation is automatic and reliable, deselecting the unused channels
and placing them in standby for stereo listening make perfect sense, and I
do so all the time. Of course, it would have been even nicer if the
Dreadnaught responded to 12V levels, not pulses, and I would have not had
to go to the trouble I did to get this working reliably, but given the
great sound with the Dreadnaught II, it was worth the effort.
After I decided to go for the RS-232 upgrade, I pulled the Dreadnaught out
of my equipment cabinet and shipped it to Theta, putting my old amplifiers
back into their previous roles. That evening we put on "Lord of the Rings,
Fellowship on the Ring", as I guessed there was nothing good on TV. As
Galadriel was narrating the introduction, my wife looked at me in shock
and said, “What happened to the sound, it's all trebly, there is no bass!" It
was true. Kate Blanchart's voice had lost most all its resonance. Even
though the subwoofer level was properly adjusted, the bass, and mid-bass
in particular seemed thin compared to the sound that we had become
accustomed to with the Dreadnaught II. The difference was not subtle.
Over the next week or so while we were without the Dreadnaught II, we
watched/listened to a variety of video material, including “The Italian
Job”, a movie a friend dropped by to watch. She thought the sound was fine
(I used to as well), but that evening I found myself constantly adjusting
the volume control trying to lock in just the right sound. I never found
it. Turning down the volume did not seem to make the sound any cleaner,
and raising the volume didn't really help that much with impact or
mid-bass.
I also noticed that the dialog was not as clear as I had become accustomed
to with the Dreadnaught II; it seemed more diffuse, like it was coming
from more than one of the speakers. Several times I walked up to, and
listened carefully to the various front speakers, but indeed the dialog
was coming only from the center channel as it should. The dialog clarity
was simply not as good with my previous setup as with the Dreadnaught II.
Needless to say, when the Dreadnaught II returned, it went back into its
proper place in the cabinet in short order. The Dreadnaught was my
amplifier now, no doubt about it – I had paid for it. While projector
technology and preamp/surround technologies are constantly improving, I'm
not too worried that the Dreadnaught will be obsolete in the foreseeable
future. There may be more efficient amplifiers out there, but they likely
won't sound any better than the Dreadnaught II. I may upgrade most of my
other components in the next few years, but I am planning of using the
Dreadnaught for the foreseeable future.
Conclusions
The Theta Digital Dreadnaught II is a very versatile amplifier and can be
configured with anywhere between one and ten channels. The five and seven-channel
configurations are ideal for a high-end Home Theater. Details
include a truly balanced differential design, with zero global negative
feedback, and Class A characteristics, leading to a very quiet, clean
sounding amplifier for both stereo and multi-channel listening. When
driving my speakers, the Dreadnaught II produced a substantial improvement
in sound quality over my previous amplifier setup. I bought a seven-channel
version for my HT system.
- Steve Smallcombe -
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